i) Field of the Invention
The present invention relates to a one-component-type composition for a polyurethane elastomer and a method for processing the same by molding. More particularly, the present invention relates to a one-component-type composition for a polyurethane elastomer which comprises as a main component a polyurethane precursor containing urethodione groups and active hydrogen groups in a specific ratio, shows excellent storage stability under ordinary environmental conditions, is cured when simply heated due to elongation and crosslinking of chains, produces no free polyisocyanate monomer during handling, has excellent handling properties, and is highly safe; and a method for processing the same by molding.
ii) Description of the Related Art
Polyurethane elastomers are excel lent in various properties such as rubber elasticity, chemical resistance, abrasion resistance, toughness, cold resistance and durability. They are therefore utilized particularly in recent years as paints, floor covering materials, water proof materials, adhesives, wall covering materials and sealing materials. Polyurethane elastomers are divided broadly into thermoplastic elastomers and thermosetting elastomers.
Thermoplastic elastomers (TPU) are linear polyurethanes having a high molecular weight, and can be processed by ordinary techniques for processing thermoplastic resins, such as injection molding or extrusion molding. On the other hand, thermosetting elastomers are classified into the following two types:
(1) Two-component type in which polyisocyanate and/or an isocyanate-terminated prepolymer and a polyol are mixed with stirring just before the use, and the mixture is then injected into a mold and cured; and PA1 (1) have the characteristic properties of conventional polyurethane elastomers, such as rubber elasticity, chemical resistance, abrasion resistance, toughness, cold resistance and durability, which result from high cohesive energy, PA1 (2) have semipermanent storage stability comparable to that of thermoplastic polyurethane elastomers, PA1 (3) have excellent heat resistance and compression set due to a crosslinked structure which is formed by a curing reaction, PA1 (4) moldability by a conventional technique for processing thermoplastic resins, such as injection molding or extrusion molding, and PA1 (5) from the environmental point of view, avoid production or splashing of free polyisocyanate monomer upon handling, i.e., blending and heating; and to provide a method for processing the composition by molding.
(2) One-component-moisture-curing type in which an isocyanate-terminated propolymer is cured by allowing it to react with moisture in the air.
These elastomers have both merits and demerits. The thermoplastic elastomers are linear polyurethanes having a high molecular weight, and have semipermanent storage stability and good working properties. However, their softening points are low because they do not have a crosslinked structure. This brings about a drawback in that they are poor in both heat resistance and compression set. Furthermore, they usually have a high molecular weight, which causes high melt viscosities and low fluidity. Therefore, they also have a shortcoming in that it is difficult to mold them into complicated shapes or large-sized articles In contrast, the two-component-type elastomers show high fluidity right after the two components are mixed, and-they can be readily molded into complicated shapes or large-sized articles. In addition, since they usually have a crosslinked structure, they are excellent in heat resistance and compression set. However, the viscosity of the mixture of the two components is increased due to a reaction between isocyanate groups and hydroxyl groups which proceeds in the system, and the mixture is finally gelled. There is therefore a limitation on the pot life of the mixture. Moreover, in order to obtain a reaction product which is satisfactorily uniform, it is necessary that the two components be used in stoichiometric amounts. It is therefore required to accurately weigh the two components and to thoroughly mix them to give a uniform mixture. On-the other hand, the one-component-moisture-curing-type elastomers are usually subjected to a process in which they are allowed to react with moisture in the air for curing. For this reason, the elastomers finally obtained have physical properties which are not so good as those of the two-component-type elastomers although their heat resistance is excellent. Since the component is reacted with moisture, the reaction proceeds slowly and requires a long time for complete curing. Furthermore, the thermosetting elastomers of the above two types contain free polyisocyanate monomers in their systems although the amount thereof is small, and the monomers may adversely affect the working environment depending on the conditions under which the elastomers are processed by molding. For this reason, it is often required to install a local exhaust system or the like.
Various proposals have been made in order to improve the above-described polyurethane elastomers. For instance, with respect to the two-component type elastomers, a method is proposed in which blocked isocyanate obtained by using a blocking agent and a polyol are made into a composition of an apparently one-component type. When the blocked isocyanate is heated to a temperature of 120.degree. C. or higher, the blocking agent is dissociated and scattered to reproduce isocyanate groups. Although the storage stability can be improved by this method, some other problems are still unsolved, for example, the elastomer is foamed due to the blocking agent when cured; environmental pollution caused by the blocking agent splashed; and the elastomer has poor properties because of the blocking agent remaining therein. Such an elastomer is therefore partly used as a one-component-type paint system in the field of paints, but scarcely used in the field of elastomers. On the other hand, with respect to the one-component-moisture-curing-type elastomers, there are proposed a formulation which is not much affected by external conditions obtained, for example, by utilizing a water-absorbing composition or by adding a catalyst; and a composition of an apparently one-component type, composed of a curing agent which produces an amino group such as a ketimine or aldimine compound when reacted with moisture, and an isocyanate-terminated prepolymer system. However, an ideal one-component-type elastomer which is excellent in long-term storage stability, working properties, and elastomeric properties is not yet developed, and thus the development of such an elastomer is now strongly demanded.
Further, the development of compositions for polyurethane elastomers which do not contain free isocyanate monomers is also strongly demanded from the viewpoint of world-wide environmental problems.
It is noted that polyurethane resins obtained by using as one of the starting materials polyisocyanate having urethodione groups are known by the following patent publications: Japanese Patent Publication No. 46447/1982, Japanese Laid-Open Patent Publications Nos. 37022-1983 and 265970/1988, Japanese Patent Publication No. 50265/1989, Japanese Laid-Open Patent Publication No. 620/1990 and Japanese Patent Publication No. 16332/1990 and the like.